Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same. In particular, the present invention is suitable for image pickup apparatuses, such as a digital camera, a video camera, a television camera, a monitoring camera and a camera for silver halide photography.
Description of the Related Art
An image pickup optical system included in an image pickup apparatus requires to be a zoom lens that has a small lens total length (the length from a first lens surface to an image plane), high resolution, and a high zoom ratio. To achieve high optical characteristics of such a zoom lens, correction of monochromatic (single wavelength) aberrations, such as spherical aberration and coma, are required to be corrected and, additionally, chromatic aberration is also required to be appropriately corrected. In particular, to achieve high resolution, a telephoto zoom lens that has a high zoom ratio and a long telephoto focal length requires that chromatic aberrations are corrected such that primary achromatism is achieved and furthermore the secondary spectrum is appropriately corrected.
Among zoom lenses having high zoom ratios, what is called a positive lead type zoom lens has been known. In this zoom lens, a lens unit at the most object side has a positive refractive power. Japanese Patent Application Laid-Open No. 2010-32702 discloses a five-unit zoom lens that consists of five lens unit, which are first to fifth lens units, having positive, negative, positive, negative and positive refractive powers in order from the object side to the image side. Japanese Patent Application Laid-Open No. 2007-163964 discloses a five-unit zoom lens that includes an optical element made of a material having abnormal dispersion. The zoom lens disclosed in Japanese Patent Application Laid-Open No. 2007-163964 includes an optical element made of an optical material having high abnormal dispersion, which reduces occurrence of chromatic aberration.
Increase in high zoom ratio in the positive lead type zoom lens increases chromatic aberrations, such as axial chromatic aberration and lateral chromatic aberration, in the entire zoom range. To achieve a high zoom ratio and high optical characteristics over the entire zoom range in the positive lead type zoom lens, reduction in chromatic aberration is important. To reduce chromatic aberration, adoption of a lens made of low dispersion and abnormal dispersion at an appropriate position in the zoom lens is effective.
In particular, it is important for the positive lead type five-unit zoom lens to appropriately adopt the lens configuration of a second lens unit having a negative refractive power for variable magnification, and materials of lenses configuring the second lens unit.